The present invention relates to optical monochromators, and more particularly, to monochromators used in applications where the timing information of the optical signals selected by the monochromator is important.
Optically-based communications based on dense wavelength division multiplexed (DWDM) signaling are becoming increasingly common because such systems greatly increase the capacity of the optical fibers used for transmitting data and voice traffic. In such systems, multiple signals are sent on an optical fiber by modulating optical signals at slightly different wavelengths. Hence, a single optical fiber can provide tens or even hundreds of communication channels that are independent from one another.
Optical telecommunication systems must transmit data and voice traffic in a manner that meets a variety of transmission specifications including jitter, rise time, fall time, and overshoot, among others. Optical filters or demultiplexers are used to select one channel from the DWDM telecommunication system for analysis by subsequent electronic test equipment such as a digital communications analyzer or a bit error ratio tester. These optical filters must not add any time dispersion to the signal to be measured. Time dispersion acts to smooth out any optical transitions degrading the measured performance of the signal being tested.
A number of optical filter designs have been used for selecting a single channel. These include the use of Fabry-Perot interferometers, thin film interference filters, Michelson interferometers, Dragone routers, and diffraction gratings. Prior art grating based optical filters inherently introduce time dispersion because the grating in general is tilted with respect to the incident light. Thus one part of the incident light travels a further distance than other parts of the light, and the transmitted signal wavefront is dispersed in time. For example, if one side of the beam travels 30 mm further than the other side, then a 10 GHz modulated signal (whose wavelength is 30 mm) would have a full wave of dispersion spread across the beam.
The HP 71452B analyzer avoids this dispersion by reflecting the focused light back towards the collimating lens and grating in a new manner for a second pass. This arrangement is discussed in detail in U.S. Pat. No. 5,233,405 to Wildnauer. The reflection flips the two sides of the beam perpendicular to the axis of rotation of the grating such that the longer traversing beam traverses the shorter path on the second pass and vice versa. This second pass also condenses the wavelength dispersion providing no additional wavelength filtering by the grating. While this design eliminates time dispersion for a narrow optical resolution bandwidth it requires relatively large lenses between the grating and the reflector used to send the beam back to the grating along a different optical path. Suitable lenses significantly increase the cost of this system.
Broadly, it is the object of the present invention to provide an improved optical analyzer that corrects for time dispersion while not requiring the large lenses discussed above.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The present invention is an optical filter for generating a filter output signal from a filter input signal, the filter output signal consisting of light from the filter input signal in a predetermined bandwidth at a center wavelength. The filter includes first and second dispersive elements and a first optical assembly. The filter provides a first signal path over which a portion of the filter input signal travels and is diffracted from the first dispersive element to form a first intermediate beam that is input to the first optical assembly. The first optical assembly generates a second intermediate beam that is directed to the second dispersive element and is diffracted by the second dispersive element. A portion of the diffracted second intermediate beam forms a portion of the filter output signal. The first intermediate beam is dispersed in time relative to the filter input signal and the second intermediate beam is dispersed in time relative to the input signal, however, the filter output signal has less time dispersion relative to the filter input signal than the first intermediate beam has relative to the filter input signal. The dispersive elements are preferably part of one or more optical gratings. The second intermediate beam is an inverted image of the first intermediate beam. The inverted image is chosen such that any time dispersion introduced by the reflection of the second intermediate beam with the second dispersive element compensates for time dispersion introduced in the first intermediate beam by the reflection of the portion of the filter input signal by the first dispersive element. Embodiments of the first optical assembly can be constructed from first and second imaging elements, each imaging element collimating light from a light source placed at a focal point associated with that imaging element. The focal point of the second imaging element is coincident with the focal point of the first imaging element. An aperture place can be included in the first optical assembly, an aperture located proximate to the focal point of the first optical element.
In another embodiment of the invention, the optical filter of claim aa1 further includes a polarization-dependent beam splitter and a half-wave plate, the polarization-dependent beam splitter having an input port for receiving the filter input signal and first and second output ports. The polarization-dependent beam splitter generates a first input light signal and a second input light signal from the filter input signal. The first input light signal is linearly polarized in a first polarization direction, and the second input light signal is linearly polarized in a second polarization direction that is orthogonal to the first polarization direction. The first input light signal leaving the first output port and the second input light signal leaving a second output port, the first and second output ports being spatially separated. The first input light signal traversing the optical signal path traversed by the second input light signal, but in the reverse direction. The half-wave plate rotates the polarization of the first input light signal by 90 degrees prior to the first input light signal being diffracted by the grating. The portion of the diffracted second intermediate beam that forms a portion of the filter output signal enters the second output port of the polarization-dependent beam splitter. A portion of the second input optical signal traverses the filter via the optical signal path, but in the opposite direction, to form a second output light signal that enters the first output port of the polarization-dependent beam splitter and is combined with the first output light signal to form the filter output signal. In the preferred embodiment of the present invention, the first optical assembly includes a reflector assembly for causing light traversing the optical signal path to be diffracted additional times from the grating such that the first and second intermediate beams have cross-sections that are substantially circular. The reflector assembly is preferably movable such that the position of the reflector assembly determines the filter center wavelength.